1. Field of the Invention
This invention relates to apparatus responsive to arc faults and particularly to such apparatus which responds to the step changes in current produced when an arc is struck, but which discriminates against the step changes in current produced by a dimmer, and especially a dimmer used with a tungsten bulb that produces a large transient in dimmer current at cold turn on.
2. Background Information
Arc faults can occur in electrical systems, for instance between adjacent paired conductors, between exposed ends of broken conductors, at a faulty connection where carbon deposits have collected on adjacent terminals or outlets, and other situations where conducting elements are in close proximity. Arc faults in ac systems can be intermittent; however, arcs caused by carbon deposits can occur regularly in successive half cycles.
Arc faults typically have high resistance so that the arc current is below the instantaneous or magnetic trip threshold of the conventional circuit breakers. Also, the intermittent nature of an arc fault can create an rms current value that is below the thermal or delayed trip threshold for such circuit breakers. Even so, the arcs can cause damage or start a fire if they occur near combustible materials. It is not practical to simply lower the pick-up currents on conventional circuit breakers as there are many typical loads which draw similar currents and which would, therefore, cause nuisance trips.
Much attention has been directed to trying to distinguish arcing currents from other intermittent currents. One approach recognizes that arc faults can generate a step increase in current when the arc is struck. However, many typical loads generate a similar step increase when a device is turned on. In many cases, the step increase produced by a load is a singular event and can be distinguished from an arc fault which generates repetitive step increases by counting step increases during an interval such as a few half cycles. A more sophisticated variant of this type of arc fault detector maintains a time-attenuated accumulation of step increases in current and generates a trip when a selected level of the accumulation is reached. This type of detector provides a faster trip on large step increases while reducing nuisance trips.
A dimmer circuit provides unique problems for an arc fault detector which responds to the step increases generated by the striking of an arc. A dimmer, when phased back, produces a pattern of step increases in current each half cycle. Under steady state conditions, the amplitude of this pulse will be below that of an arc current, and can, therefore, be distinguished on that basis. However, if the dimmer is used to control a tungsten bulb, the cold filament has a very low resistance on startup, which produces a large initial pulse with subsequent pulses decaying in amplitude as the filament rapidly warms up. This characteristic of a dimmer has also been used to distinguish it from arc faults, but it still has been necessary to maintain the threshold for arc detection well above the handle rating to avoid nuisance tripping on a dimmer.
As mentioned, carbon arc faults can strike in successive half cycles and thus look very similar to a dimmer. This presents a difficult challenge in meeting code requirements for a reliable response to carbon arcs while rejecting nuisance trips in response to a dimmer including turn-on of a cold tungsten bulb.
Known prior art apparatus which seeks to provide immunity to dimmers energizing tungsten bulbs attempt to reduce the sensitivity of the detector to the dimmer transient at cold turn-on.
There is room therefore for improvement in detectors and circuit breakers providing protection in electrical circuits which are subject to the transient currents generated by cold turn-on of a tungsten bulb by a dimmer.
The present invention rather than just reducing the sensitivity to transients caused by dimmers energizing a tungsten bulb during cold turn-on, disables the arc fault detector under these conditions to provide complete immunity to the phenomenon. The invention is applicable to arc fault detectors and to circuit breakers incorporating arc fault detectors which respond to pulses generated for each step change in current in the ac electrical circuit. The response means generating the arc signal is disabled when the pulses track within a predetermined margin the pulses generated by a dimmer under certain conditions such as those created by a dimmer energizing a cold tungsten bulb.
More particularly, the invention relates to apparatus for detecting arc faults generating step changes in current each time an arc is struck in an ac electrical circuit having a dimmer generating in successive half cycles step changes in current which, under certain transient conditions, produce a dimmer transient current in which the step changes in current decay in amplitude in successive half cycles approximately exponentially from an initial transient value to a steady state value. The apparatus comprises pulse generating means for generating a pulse signal having a pulse for each step change in current in the ac electrical circuit, and response means responsive to repeated pulses generated by the step changes in current above the steady state value for generating an arc signal. The apparatus further includes disabling means responsive to the pulse signal for preventing the response means from generating the arc signal when the pulses in the pulse signal track the dimmer transient current within a predetermined margin.
In accordance with a preferred embodiment, the disabling means comprises a peak detector generating a decaying peak signal which is initially proportional to the initial value of the step change in the dimmer transient current under these certain conditions and which then decays, preferably approximately exponentially. The disabling means can include tracking means for tracking the decaying peak signal and generating a disable signal only so long as the amplitude of the pulse signal remains less than an upper percentage of the decaying peak signal and more than a lower percentage of the decaying peak signal. Also preferably, the tracking means includes a first timer blocking generation of the disable signal until the pulse signal has remained less than the upper limit for a selected time period. This time period can be a predetermined number of half cycles of the ac current, for example, at least three half cycles. The tracking means can also include a second timer blocking generation of the disable signal when the pulse signal does not exceed the lower percentage of the decaying peak signal during each half cycle.
The peak detector can also include a reference signal generator generating a first reference signal which is the upper percentage of the decaying peak signal and a second reference signal which is the lower percentage of the decaying peak signal. In this case, the tracking means can include a first comparator comparing the pulse signal to the first reference signal, a second comparator comparing the pulse signal to the second reference signal and logic means generating the disable signal when the first comparator indicates that the pulse signal is less than the first reference signal and the second comparator indicates that the pulse signal is more than the second reference signal. Where the logic is digital, the tracking means can include blocking means for delaying the first comparator from setting the digital logic for generating a disable signal until the pulse signal remains below the first reference signal for a predetermined number of half cycles. The tracking means can also include second blocking means setting the logic to block the generation of the disable signal when the pulse signal does not exceed the second reference signal during a half cycle.
The invention also embraces a circuit breaker incorporating an arc fault detector as described.